1. Field of the Invention
This invention relates to a stage assembly, a support system, and method for stabilizing the stage assembly in a photolithography process to manufacture semiconductor wafers. More particularly, this invention relates to the stage assembly, support system, and method for stabilizing the stage assembly to prevent ground vibration.
2. Description of the Related Art
In manufacturing integrated circuits using photolithography, light is transmitted through non-opaque portions of a pattern on a reticle, or photomask, through a projection exposure apparatus, and onto a wafer of specially-coated silicon or other semiconductor material. The uncovered portions of the coating, that are exposed to light, are cured. The uncured coating is then removed by an acid bath. Then, the layer of uncovered silicon is altered to produce one layer of the multi-layered integrated circuit. Conventional systems use visible and ultraviolet light for this process. Recently, however, visible and ultraviolet light have been replaced with electron, x-ray, and laser beams, which permit smaller and more intricate patterns.
As the miniaturization of a circuit pattern progresses, the focus depth of the projection exposure apparatus becomes very small, making it difficult to align accurately the overlay of circuit patterns of the multi-layered integrated circuit. As a result, a primary consideration for an overall design of the photolithography system includes building components of the system that achieve precision by maintaining small tolerances. Any vibration, distortion, or misalignment caused by internal, external or environmental disturbances must be kept at minimum. When these disturbances affect an individual part, the focusing properties of the photolithography system are collectively altered.
In a conventional exposure apparatus of a photolithography system, a wafer stage assembly is used in combination with a projection lens assembly to manufacture semiconductor wafers. The wafer stage assembly includes a wafer table to support the wafer substrates, a wafer stage to position the wafer substrates as the wafer stage is being accelerated by a force generated in response to a wafer manufacturing control system, and a wafer stage base to support the wafer stage. The wafer manufacturing control system is the central computerized control system executing the wafer manufacturing process. To permit smaller and more intricate circuit pattern, the projection lens assembly must accurately focus the energy beam to align the overlay of circuit patterns of the multi-layered integrated circuit.
The conventional exposure apparatus generally includes an apparatus frame which rigidly supports the wafer stage assembly, the projection lens assembly, the reticle stage assembly, and an illumination system. In operation, the exposure apparatus transfers a pattern of an integrated circuit from a reticle onto the wafer substrates. The exposure apparatus can be mounted to a base, such as the ground or via a vibration isolation system.
There are several different types of photolithography devices, including a scanning type and a step-and-repeat type. In the scanning type photolithography system, the illumination system exposes the pattern from the reticle onto the wafer with the reticle and the wafer moving synchronously. The reticle stage moves the reticle on a plane which is generally perpendicular to an optical axis of the lens assembly, while the wafer stage moves the wafer on another plane generally perpendicular to the optical axis of the lens assembly. Scanning of the reticle and wafer occurs while the reticle and wafer are moving synchronously.
Alternately, in the step-and-repeat type photolithography system, the illumination system exposes the reticle while the reticle and the wafer are stationary. The wafer is in a constant position relative to the reticle and the lens assembly during the exposure of an individual field. Subsequently, between consecutive exposure steps, the wafer is consecutively moved by the wafer stage perpendicular to the optical axis of the lens assembly so that the next field of the wafer is brought into position relative to the lens assembly and the reticle for exposure. Following this process, the images on the reticle are sequentially exposed onto the fields of the wafer.
Regardless of the type of photolithography system being used, to focus accurately the image transferred from the reticle onto the wafer, the exposure apparatus must align a position of an exposure point on the wafer with a position of the focal point of the projection lens assembly.
To maximize throughput of wafer production, the reticle stage and the wafer stage must move at high acceleration rates. To generate high acceleration rates, the force generating motors must produce large stage forces Fin over short durations to move the reticle stage or the wafer stage, such as diagrammatically shown in FIG. 2A. The stage forces move either the reticle stage or wafer stage according to the graph shown in FIG. 3A.
According to Newton's second law, these types of impulses generate reaction forces on the base, which cause the reticle stage base or a wafer stage base to move according to the graph shown in FIG. 3B. Since both the reticle base and wafer stage base are rigidly connected to the apparatus frame of the exposure apparatus, the reaction forces are transmitted to the apparatus frame and the ground, causing a detrimental vibration to the photolithography system.
Therefore, there is a need for an improved stage assembly, stage support system, and method to eliminate or substantially reduce the vibration.